1. Field of the Invention
This invention relates to improvements in indicators such as the type used to display flight navigation information and more particularly to electronic circuitry used in conjunction with such indicators to provide signals to operate such indicators in predetermined manners within certain predefined ranges.
2. Description of the Prior Art
In the navigation equipment art, it has been practiced typically to provide an electric meter indicator to display course deviations. For example, in an instrument low-approach system (ILS) indicator, glideslope and localizer signals are provided from which the relative aircraft position can be determined. For example, typically an electric meter in the cockpit indicates the position of the aircraft above or below a glideslope path defined by patterned electromagnetic energy lobes transmitted from an antenna adjacent the end of the runway. The glideslope indicator indicates the deviation of the aircraft above or below the glideslope by meter deviations above or below a centered position. When the aircraft is within a predetermined distance from the glideslope signal antenna just prior to landing, the indicator variations are linear with deviation from the glideslope course. On the other hand, when the aircraft is beyond a predetermined range from the runway, typically a flag is maintained clearly indicating to the pilot that the information displayed on the indicator cannot be relied upon. In some devices, although not typically, the indicator may be positioned in a "fly up" or other predetermined position.
In addition, recently, in certain circumstances, aircraft warning indicators have desirably been driven completely off of the indicator scale. This is desirable, for instance, especially when the received signal is insufficient to reliably indicate a course upon which the aircraft should be flown, or, if no signal at all is received by the navigation receiver. Such out-of-view operation has been achieved by the use of secondary springs in electric meter indicators. Secondary springs, however, are difficult to adjust, requiring highly skilled maintenance technicians, and, may additionally affect the operation of the meter in its intended response over a linear indication range.
Additional means have been used to drive the indicator meter off scale in the form of operational amplifiers whose positive output is limited to keep the pointer partially in view. A discrete second signal (such as a "loc mode" in a glideslope application) is then removed to drive the indicator out of view. This, of course, has disadvantages in the need for a discrete second signal, which may not be present in certain "lost signal" conditions, and, in addition, requires that the operational amplifiers be carefully controlled to insure little drive variations.